Advanced gas turbine engine components operating into the hot sections of the engine can experience temperatures which can contribute to reduction in the operating life of the components. Generally, such components include combustors, turbine blades and vanes, turbine shrouds and various components in the exhaust system. To improve component life, such components have been designed to include cooling passages within the body of the component exiting a surface or about a component surface. For additional thermal and/or environmental resistance, frequently there is included on a surface exposed to higher temperatures, a high temperature protective coating such as a metallic or ceramic type coating. Examples of such coatings, widely reported in the art, include aluminides, MCrAlY (in which M is at least one of the elements Fe, Co, and Ni), various metal overlay type coatings, and ceramic thermal protective coatings, one type of which is referred to in the art as Thermal Barrier Coatings or TBC. Although the present invention relates to all such coatings, of particular interest are the TBC coatings.
Typical of TBC coatings, for example as used in the gas turbine art, is one based on zirconia stabilized with yttria, for example about 92 wt. % zirconia stabilized with about 8 wt. % yttria. A preferred method for application of a TBC coating is by plasma spray using apparatus sold commercially for that purpose. Frequently, such coatings are used with a metallic bond coat. This general type of TBC coating has been reported for some time as evidenced by such U.S. patents as U.S. Pat. No. 4,055,705--Stecura et al. (patented Oct. 25, 1977); U.S. Pat. No. 4,095,003--Weatherly et al. (patented Jun. 13, 1978); U.S. Pat. No. 4,328,285--Siemers et al. (patented May 4, 1982); U.S. Pat. No. 5,216,808--Martus et al. (patented Jun. 8, 1993); and U.S. Pat. No. 5,236,745--Gupta et al. (patented Aug. 17, 1993). The disclosure of each of these patents hereby is incorporated herein by reference.
The above incorporated Martus et al. patent shows one configuration of a fluid cooled, in this example air cooled, gas turbine engine turbine blade. Such blade has a hollow interior connected through cooling passages, such as cooling channels or holes through the wall, to the outer wall surface. Protecting the outer surface, in one form of the invention, is a TBC coating. Because the cooling holes are constructed of a size to provide proper or desired cooling of the blade during engine operation, reduction in fluid flow through the holes should not be restricted as a result of such a surface application. Therefore, Martus et al., in one form of their invention, teach removing from any cooling hole excess material, for example TBC, obstructing flow through the hole. It has been found that, in some cases, laser drilling after TBC coating is unsatisfactory because of cracking at the interface between the TBC and its substrate. Although Martus et al. describe a method for removing undesirable flow blockage, eliminating the need for such a removal step would reduce risk of interface cracking, and would lower manufacturing cost and increase manufacturing efficiency.